1. Field of the Invention
The present invention relates to a method and apparatus for handling generally planar objects during multistage processing steps, and specifically, a processing cassette having a flexible structure which is compliant rather than rigid.
2. Description of the Related Art
A variety of high technology products are currently formed by subjecting a surface of a generally planar object to a number of sequential processing steps. Examples of products processed in this manner include optical and magnetic recording disks, photomasks, and silicon wafers from which semiconductor chips are fabricated. For the sake of simplicity, the present invention is discussed below with reference to the manufacture of semiconductor devices from silicon wafers. However, this is not intended to limit in any way the scope of application of the apparatus or method of the present invention.
Surface processing of silicon wafers to make semiconductor devices such as integrated circuits involves a number of stages, with the wafers being exposed to many different processing steps. Wafers are placed in tanks at work stations where they are repeatedly subjected to heated chemicals, such as sulfuric acid. The wafers are also rinsed and cleaned between processing steps. For optimum process performance, it has become customary for the wafers to be confined within specially-constructed relatively low mass cassettes having customized interfaces with the tanks into which the cassettes are placed, and with the robots or other machinery for transporting the cassettes.
In general, the wafers are arranged in a linear array configuration within the cassette, with sufficient space between each wafer to allow exposure to the various chemicals necessary for processing, and to prevent contact with adjacent wafers. The cassette is typically rigid in order to maintain the required spacing between the wafers, even under the relatively high temperatures to which the cassette is exposed.
One of the most common wafer cassette designs currently used is the rigid four-bar cassette shown in FIGS. 1A-1C. Such a prior art cassette is described in U.S. Pat. No. 4,872,554. FIGS. 1A-1C show perspective, top, and end views, respectively, of a conventional rigid four-bar wafer cassette. Rigid endplates 2 secure the ends of each of the rigid bars or rods 3. Rigid bars 3 have combs or "teeth" 4 adapted to accept and separate inserted wafer(s) 5. An inserted wafer is intended to come into contact with cassette 1 at four different points 6, as shown in FIG. 1C. The wafer's position is statically indeterminate, and in practice wafer 5 typically maintains contact with combs 4 on only three of the four bars of the conventional cassette.
Because the conventional four-bar structure is rigid, the cassette must be loaded and transported facing upwardly, otherwise the wafers fall out. Also, the separation of the wafers within the cassette (called the wafer "pitch") is greater than is desired because when such a conventional cassette is loaded into a tank of hot liquid, thermal expansion causes the cassette to expand so that the wafers become "looser" and tend to lean. Thus additional space must be provided between the wafers to ensure that they do not contact each other.
The traditional four-bar cassette of FIGS. 1A-1C is picked up and transported using handles 7. This requires a relatively complicated gripping arrangement to transport a cassette. The four-bar cassette design has other disadvantages which will become apparent in the description of the present invention which follows.
Wafer cassettes have been created using a number of different materials, and for the purposes of the invention the material from which the cassette is made is not critical. That is, once the compliant cassette concept of the invention is understood, those skilled in the art will readily understand how such a cassette could be formed from any number of materials. I have found, however, that currently the most desirable material for wafer carriers is PFA (perfluoro-alkoxy) Teflon brand material (hereinafter referred to for simplicity as PFA). PFA exhibits high resistance to a broad variety of chemicals, and is not readily contaminated by other materials. PFA is thus preferred for ultra clean applications such as the fabrication of semiconductors. However, PFA does have less than optimal structural properties and a high coefficient of thermal expansion.
Alternative materials used for rigid process cassettes include fused quartz. Unfortunately, fused quartz is fragile and susceptible to etching by some chemicals commonly utilized in wafer processing.